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Bacterial concentrations in pus and infected peritoneal fluid--implications for bactericidal activity of antibiotics
C König1, H P Simmen, J Blaser
1Department of Medicine, University Hospital Zurich, Switzerland.
Abstract:
Little is known about how many bacteria are present at an infectious focus at the onset of antibiotic therapy. The number of cfu was determined in pus and infected peritoneal fluids obtained from 41 patients. Pathogens were detected in 71% of specimens. There were high concentrations of bacteria in culture-positive samples, in both soft-tissue and peritoneal infections, averaging 2 x 10(8) cfu/mL. These concentrations were much higher than the standard inoculum size used in in-vitro susceptibility tests, 5 x 10(5) cfu/mL. The impact of this discrepancy on antibacterial efficacy was studied with amikacin, ciprofloxacin, imipenem and piperacillin against Escherichia coli and Staphylococcus aureus. The inhibitory and bactericidal activities of amikacin and ciprofloxacin determined with high inocula were two to four times lower than with standard inocula, whereas the activity of piperacillin was diminished at least 128-fold. Similar activity was observed with these drugs in Mueller-Hinton broth and peritoneal fluid. The bactericidal activity of imipenem was reduced in peritoneal fluid. Thus, conditions prevailing at the infection site may compromise antibiotic activity determined in vitro.
Insights
Bacterial infections often have high bacterial concentrations (colony-forming units, cfu) at the infection site, far exceeding standard laboratory test levels. This significantly reduces the in vitro effectiveness of certain antibiotics against common pathogens.
Area of Science:
- Microbiology
- Infectious Diseases
- Pharmacology
Background:
- The bacterial load at infection sites at the start of antibiotic treatment is largely unknown.
- Standard in vitro antibiotic susceptibility testing uses a lower bacterial concentration (inoculum size) than typically found in clinical infections.
Purpose of the Study:
- To quantify bacterial concentrations in clinical infection samples (pus and peritoneal fluid).
- To evaluate the impact of high bacterial concentrations on the in vitro activity of key antibiotics against common bacterial pathogens.
Main Methods:
- Collected pus and infected peritoneal fluid samples from 41 patients.
- Determined bacterial concentrations (colony-forming units, cfu/mL).
- Assessed the in vitro inhibitory and bactericidal activity of amikacin, ciprofloxacin, imipenem, and piperacillin against Escherichia coli and Staphylococcus aureus using both standard (5 x 10^5 cfu/mL) and high (approx. 2 x 10^8 cfu/mL) inocula.
Main Results:
- Pathogens were detected in 71% of samples, with high bacterial concentrations averaging 2 x 10^8 cfu/mL.
- Antibiotic activity was significantly reduced at high bacterial concentrations: amikacin and ciprofloxacin showed 2-4 fold lower activity, piperacillin was reduced at least 128-fold.
- Imipenem's bactericidal activity was reduced in peritoneal fluid compared to standard broth.
Conclusions:
- Clinical infection sites harbor bacterial loads substantially higher than those used in standard in vitro antibiotic susceptibility testing.
- These high bacterial concentrations can significantly compromise the in vitro efficacy of several important antibiotics, potentially impacting clinical outcomes.
- In vitro antibiotic testing should consider simulating conditions closer to the in vivo infection environment for more accurate predictions of treatment success.